In universal motors, particularly in electric tools, there is the danger of overheating through excessive loading which can lead to irreversible motor damage.
This danger of overheating is usually countered by switching the motor current off, if a temperature limit is exceeded, or by reducing said current to values presenting no danger. For this the temperature of the motor winding must either be directly measured by means of a temperature sensor, for instance, a positive temperature coefficient (PTC) resistance wound into the winding or it must be indirectly determined from magnitudes which can be handled by measurement technology, for instance, by means of a cooling shunt.
The direct measuring method has the advantage of high precision. It is, however fraught with a high expenditure of additional component costs, installation work and separate wiring. Indirect methods avoid this effort as a rule. They have, however, the disadvantage of being somewhat imprecise and, above all, they cannot reproduce the true motor temperature with sufficient accuracy, above all, because of inherent time delays.
With direct measuring methods, such as those with a measuring sensor or sensors, there are known, for instance, heat sensor elements such as positive temperature coefficient- and negative temperature coefficient resistors or thermal contacts attached to the motor windings (mostly at the or the plurality of the starter windings), whose respective output signals are evaluated.
Measuring methods without measuring sensors are also used as direct measuring methods, which include a known, if however, not very widespread method, to measure the ohmic resistance of at least one motor winding, which constitutes a nearly directly proportional measurement for its temperature. Resistor measurement methods of this type are known in many variants and are described in the professional literature. It is, however, difficult to apply this method to AC motors, because in this case, ohmic resistance must be differentiated from the AC current impedance.
In the indirect temperature measuring method, equipment-modifying, as well as computerized methods, are known.
With the equipment-modifying methods, it is attempted to determine the motor temperature by suitable cross-switched electronic and/or electric components, from which a signal corresponding to the motor temperature can be tapped. Usually, the current flowing through the motor and/or the applied voltage is processed for this purpose. The simplest and mostly widespread type is the use of a thermal circuit breaker, which is wired in series with the motor and which is heated by the motor. When a limit temperature is exceeded, a contact opens and interrupts the current circuit.
In this connection, a method is, for instance, also known to the effect of having the motor current flow through a heating resistance, which is in thermal contact with a temperature sensitive component and which is dimensioned, in such a way, that the absolute value as well as the chronological course of its temperature corresponds to the best possible degree to the motor temperature. By arranging the heating resistance in the motor cooling airflow, another refinement of the method can be achieved.
In the computerized methods, motor temperature is, as a rule, reconstructed with the help of a thermal model. Herein, easily measured magnitudes, such as voltage, current, rpm and/or phase operating angles are processed in such a way by means of a digital computer (frequently a micro-controller), that output data is indicative of the motor temperature. In this connection, methods are known which utilize empirically determined, motor specific, stored characteristic field tables (compare DE 31 11 818 A1), as well as those which compute closed mathematical models (compare DE 31 07 123 A1).